How Your Fertility Works
These pages give a simple explanation of how male and female fertility work and
complement each other, enabling couples to plan, space and limit family size,
without using any invasive form of contraception.
A few “Fertility Facts” to think about:
A man is fertile all the time,
producing millions of sperm
at each act of intercourse
A woman, by contrast, usually produces
one egg per monthly cycle which can be fertilised
for about 12 hours
Sperm can live for 3 - 6 days before ovulation,
given good conditions
So conception can occur from intercourse up to 6 days before ovulation.
Allowing a further 3 days for the release and life span of the egg,
The rest of the cycle is infertile.
Understanding the above facts helps you to see how precise the fertile window is in
each cycle. This tutorial explains the natural signs and symptoms in a woman’s cycle
which show her when she is fertile and when she is not. It draws all these signs
together to provide a means of family planning which is effective, yet has no health
risks or side-effects. It is often called Fertility Awareness, or more commonly,
Natural Family Planning.
After reading this tutorial, if you wish to learn more about charting your cycle,
please down load the tutorial “HOW TO CHART CYCLES”.
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A simple view of male fertility
A man begins to produce sperm in early adolescence, and normally continues to do so throughout
his entire life.
In this illustration we see a simplified view
of a man's reproductive system. Inside each
testicle is a metre of tubules where sperm are
produced, at an estimated rate of a thousand
per second.
Once produced, the sperm work their way
along the tubules towards the epididymis in
each testicle, where they are stored and
matured.
During intercourse they are
propelled rapidly out along the vas deferens
tubes, past the bladder and the seminal
vesicles.
They enter the prostate gland, where they
combine with fluids called "seminal fluid"
before travelling along the urethra and out of
the penis.
The total content of the ejaculation measures
about 5 mls. (a teaspoonful), but it can
contain between 1 - 400 million sperm.
Sperm Life—Key to Fertility
Sperm swim like tadpoles, by vigorous
movement of their tails. Once ejaculated
from the man, their life-span is affected by
acid or alkaline conditions.
In acid conditions sperm clump together
and are destroyed within minutes, maximum
life a couple of hours. (Fig. A)
In alkaline conditions they thrive, swim
vigorously and can live for several days.(Fig.
B)
It is surprising to learn that a woman’s
vagina is acidic for much of her monthly
cycle—to fight bacteria. There is only a
short time leading up to ovulation when a
unique secretion makes the vagina alkaline
and able to support sperm life. These
alkaline days are the only days when
pregnancy is possible.
Once couples learn to recognise the acid and
alkaline phases of the woman’s cycle, they
can plan their family naturally, simply by
planning or avoiding intercourse in the
alkaline fertile phase of the cycle.
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A woman’s fertility explained
This simplified side view of the pelvic area
shows the location of her reproductive
system.
The outer vaginal area is called the vulva and
three passages lead down to this area.
The first passage is the urethra, the urine
duct which leads down from the bladder.
The back passage is the rectum or bowel
passage which is the end of the intestines
leading down from the stomach.
The middle passage is the vagina where
intercourse occurs. Towards the top of the
vagina is the cervix which acts as the door
to the womb.
At the upper broad end of the womb are the
two fallopian tubes which end in delicate
fingers called fimbria. They caress the
surface of each ovary, searching for a
ripening follicle (egg sac), ready to receive
the egg when it is released at ovulation.
Front view of the womb, isolated from the structures around it.
For fertilisation (conception) to take place,
sperm have to swim from the vagina, through
the cervix, up into the womb and along the
tubes to unite with the egg in the outer third
of the fallopian tube. But the journey is full
of hazards.
The first problem they meet is vaginal
acidity, which exists for much of the monthly
cycle and destroys sperm life.
The second hurdle is the cervical os, the door
to the womb, which acts like a valve
controlling the access of sperm. For much of
the woman’s monthly cycle, it remains
tightly closed and sealed, blocking the access
of sperm, confining them to the acid vagina
where they are destroyed.
For a few days before ovulation, the cervix
opens wide and secretes an alkaline mucus
to entice the sperm upwards into the cervix so
that they can travel up into the tubes to
fertilise the egg on its release. This is the only
time pregnancy is possible. This is explained
more fully in the next two diagrams.
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The role of the cervix
To understand the important role of the cervix, we need to see the womb in cross section.
When there is no egg ripening in the
ovary, the doorway to the womb (cervical
os) is pinched shut to keep sperm out.
In addition, the passage through the cervix
(cervical canal) is blocked by a white
sticky mucus, produced from glands inside
the cervix. This mucus is highly acidic
and has a mesh structure, like a fishing
net, to trap and destroy sperm. In these
conditions, sperm cannot enter the
cervix and their life expectancy in the
vagina will be minutes, rather than
hours.
Pregnancy cannot occur when this “mesh mucus” is present,
This lower illustration shows how
everything changes once an egg is
selected in the ovary. As the egg follicle
ripens, it produces a hormone oestrogen
which is the signal for the cervix to open.
The oestrogen also stimulates the glands
in the cervix to disperse the mesh mucus
and produce instead a wet slippery
mucus with swimming lanes along
which sperm can move easily, as if on a
“motorway”.
This new mucus is
alkaline and contains glucose and other
nutrients vital to sperm survival. It
appears for approx. 6 days before
ovulation and then dries up quickly. In
this mucus, sperm can swim up into
the cervix and survive for 3—6 days.
Pregnancy can occur only when this mucus “motorway mucus”
is present
In Summary
The “mesh” mucus blocks the passage of sperm, preventing pregnancy from occurring.
The “motorway” mucus prolongs the life of sperm and enables them to swim through the
cervix and womb and into the fallopian tubes, in search of the egg.
If couples learn to recognise these mucus changes, they can use the information to plan,
space and limit their family size simply by observation and the appropriate timing of
intercourse.
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Understanding a Woman’s Cycle
For Natural Family Planning purposes, the
first day of the menstruation (period) is
counted as the first day of a new cycle.
The period (which lasts between 4–6 days
usually) is simply the unused lining of the
previous cycle being washed away amid
blood loss so that a new cycle can start.
It must be pointed out that not all “bleedings”
are menstruations. For example, in the teen
age years, or the pre-menopause years, or in
stress situations, women can bleed erratically
and mistakenly count some of these bleeds as
their usual menstruation.
A true period is a bleeding which follows
about 10 – 16 days after an ovulation.
As the period draws to an end, if the egg for
that cycle has not yet been chosen, the cervix
is pinched shut and the thick sticky “mesh”
mucus fills the cervix. The woman is then in
an infertile phase of her cycle.
How long does this phase last? It varies from
woman to woman, and even for the same
woman at different stages in her life.
Women whose cycles are very regular, may
find they have the same number of infertile
days month after month, but other women will
find variations. Some cycles will have fewer
days, others more days. Some women with
very short cycles may have none at all, but
may go straight into the fertile phase.
Careful observation and instruction is needed in this phase of the cycle to accurately
identify the genuinely infertile days at the start of a cycle.
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Onset of the fertile time
Eventually, at some point after the period,
the egg for that cycle will be chosen and its
follicle will grow very quickly in the ovary.
In approximately six days, it will be ready to
burst and shed its egg - ovulation.
(This growth spurt can be seen very clearly
with ultrasound.)
As already explained, the cervix opens and
changes the mesh mucus into “motorway”
mucus, which is alkaline and rich in
nutrients. The woman is in the fertile
phase of her cycle!
Now sperm can enter the cervix and survive
in little pockets in the walls (cervical crypts)
for several days. They are primed and given
the qualities they need to reach and fertilise
the egg. At first, the “motorway” mucus
looks cloudy and wet. Nearer ovulation, it
becomes more clear, abundant, stretchy and
slippery like raw egg-white.
The stage is now set for the high point of the cycle – ovulation!
To burst the follicle and release the egg, a
new hormone called Luteinising Hormone
(LH) is sent by the brain. (Test kits for this
hormone can be bought in pharmacies).
The released egg is sucked into the fallopian
tubes and now remains fertilisable for about
12 hours.
The ruptured follicle is not wasted. Instead,
it is re-cycled to become the Corpus
Luteum, which produces the next hormone
called Progesterone (pregnancy hormone).
Progesterone has three effects on the womb: 1. It stops any further ovulations in that cycle.
2. It thickens the mucus to seal the womb 3. It builds up the womb lining (endometrium)
ready to receive and nourish the baby, if conceived.
Every cycle is the same up to this point. Whether conception occurs or not will all
depend on whether the couple had intercourse in the fertile phase of the cycle, when the
motorway mucus was present. The next page shows a conception cycle.
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A Conception Cycle
Intercourse in the fertile phase
If a couple had intercourse in the egg
ripening, fertile mucus phase of the
cycle, the sperm will find perfect
swimming conditions from the vagina
into the open cervix, where they will
be stored in the cervical crypts.
In the crypts, they will be well fed,
matured, primed and released upwards
in regular convoys over the next few
days to travel through the womb into
the fallopian tubes in search of the
egg.
Conception and Implantation
If a sperm penetrates the crust of the
egg, then fertilisation (conception) is
achieved: – Day 1 of the new baby’s
life.
The baby’s cells divide rapidly as it is
propelled down the fallopian tube
towards the waiting womb and its
nutritious lining.
Within hours of ovulation, the fertile
mucus begins to dry up and the mesh
mucus returns to seal the womb.
By Day 8:
the baby has implanted in its mother’s womb and is sending messages to
prevent menstruation.
By Day 17: the placenta is established; the baby’s own blood cells have begun
to develop.
By Day 20: the foundation for the entire nervous system is established.
Day 21:
the baby’s heart starts to beat!
The baby’s rapid growth continues in its mother's womb, now cushioned by a fluid sac.
The mesh mucus plug continues to function for nine months, sealing the womb and
protecting the baby from infection. It is dislodged during at the onset of labour.
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The unseen life of the baby
Day 30:
There is regular blood flow within a
closed vascular system. The ears and
nose start to develop.
Day 42:
The skeleton is now complete and
reflexes are present. The liver, kidneys
and lungs are formed.
Day 42:
Electrical brain wave patterns can be
recorded.
Day 56:
All organs are functioning; growth and maturity are all that occur now, in the same way that
a child grows into an adult.
Day 65:
The baby can make a fist and will grasp an object stroking the palm of its hand.
It will also leap up and down in the womb with movements co-ordinated.
Week 16:
Baby is half its birth length, and its heart pumps fifty pints of blood a day.
Week 20:
Hair appears on the baby’s head and it weighs approx. l lb. (450g)
Week 28:
The baby can open its eyes and can hear its mother’s digestive processes and heart beat.
Week 40:
The baby emerges in labour
and birth and the fertility
cycle is complete –
a child is born.
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The cycle without pregnancy
If, however, no intercourse took place
in the egg-ripening phase, when the
fertile “motorway” mucus was being
secreted from the cervix, then the egg
cannot be fertilised.
It dies quickly and the cycle will end
differently!
The empty egg follicle will convert to a
corpus luteum and produce the
hormone progesterone which will
thicken the womb lining for about 10 –
16 days, as before.
The cervix will pinch shut again and the
mesh mucus plug will be re-installed in
the cervical canal.
Progesterone will also stop any more
eggs being released in that cycle.
Everything is the same up to this point
in every cycle.
However, this time, because no
conception occurred, the progesterone
fades away about 10-16 days after
ovulation and it all ends differently.
The thickened womb lining is not
needed because pregnancy did not
occur and it is discharged amid blood
loss over the next 4 – 6 days,
constituting the next menstruation.
So the menstruation /period marks the
end of this cycle – and becomes Day 1
of the new cycle! And the whole
process starts again ….
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Summarizing the events of the cycle
A cycle starts with the first day of menstruation. How the cycle ends will depends on
whether the couple had intercourse in the egg ripen fertile phase.
The Pregnancy Cycle
At some point after menstruation
(variable phase), an egg will be
chosen and begin to ripen. As it
ripens, the fertile “motorway” mucus
is produced to attract and feed sperm.
(top diagram)
If intercourse takes place in the fertile
phase the egg can be fertilised. The
womb lining will thicken, the cervix
will be closed and sealed with mesh
mucus. (middle diagram)
The baby can implant in the enriched
womb lining to continue its growth
and development to birth. (bottom left
diagram)
The Cycle without pregnancy occurring
If no intercourse takes place in the fertile mucus phase, the egg cannot be fertilised.
(top diagram)
The unfertilised egg dies quickly, the womb lining builds up as usual for 10—16 days and
the cervix is sealed with a thick “mesh” mucus plug. (middle diagram)
However, because no conception and implantation occurred, after 10 – 16 days the womb
lining will be discharged in menstruation. (bottom right diagram)
– and the whole process starts again with a new cycle
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Charting the fertility cycle
Cycles can be explained and charted in many different ways. This colour scheme, based on the
seasons of the year as seen in trees, helps to highlight the fertile and non-fertile phases of the cycle.
Red represents the period.
Brown (the bare winter tree) represents the
“winter” time of the cycle – the infertile time
when the “mesh” mucus plug is present,
blocking the access of sperm
Green (the green summer tree) represents
the “summer” time of the cycle - the fertile
time, when the egg is ripening and the
“motorway” mucus is being secreted.
Below are three cycle patterns displayed with this colour scheme.
a short cycle
(22—24 days)
Fertile mucus (green block) starts straight after the period, possibly even before the period has
actually stopped. The mucus continues for about 5—6 days and the last day of it is called PEAK
Day (peak of fertility) - marked P on the chart.. Ovulation occurs around PEAK Day. After a
count of 3 dry days (brown block 1,2,3) to allow for the release and lifespan of the egg, the
remaining brown days are infertile till the next period starts.
an average cycle
( 27—31 days).
A few DRY infertile days (brown block) usually follow the period. Then the fertile mucus appears
(green block) and leads up to ovulation around PEAK Day. After the count of 3 dry days after
Peak, the rest of the cycle is infertile (brown block) until the next period starts.
A long cycle
5-6 weeks +
The DRY infertile days after the period (brown block) stretch to several days (even weeks, in really
long cycles). Eventually the fertile mucus appears (green block), leading up to ovulation around
PEAK Day. After the count of 3 dry days after PEAK, to allow for the release and life span of the
egg, the rest of the cycle is infertile (brown block).
For those trying to achieve pregnancy, the PEAK Day (the last day of the fertile mucus i.e. last day
of green block) is the most fertile day of the cycle!
The above charts are not intended to be used alone. Please see subsequent pages to learn how
to double check symptoms using the Temperature and other indicators.
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Understanding the hormone pattern of the cycle
None of the events of the cycle happen by chance. They are all controlled by chemical
messengers called hormones. The two main hormones in a woman’s cycle – oestrogen and
progesterone - are explained in the diagram below.
1. Oestrogen comes in
increasing amounts from the
ripening egg follicle and
causes the cervix to open and
secrete fertile “motorway”
mucus till the egg is ready for
release. The last day of fertile
mucus is called Peak Day–
peak of fertility
2. The dotted vertical line
represents LH (luteinizing
hormone) which triggers
ovulation.
3. The empty follicle converts
to a yellow structure, called
the corpus luteum, and it
produces
the
dominant
hormone progesterone which:
 stops all further ovulation
in that cycle.
 glues up the mucus,
 builds up the womb lining,
Allowing three days after Peak
Day, for the mucus to thicken,
all the remaining days of the
cycle are infertile until the
start of the next period.
The event of ovulation can also be confirmed by another sign – a rise in body
temperature. This well researched method has been used world-wide with great success
since the 1950’s, as a natural means of family planning.
Effects of the fertility cycle on body temperature
Before ovulation a woman’s body temperature is normal, because oestrogen has no effect
on body temperature.
However, after ovulation, progesterone in her body causes her temperature to rise and
remain high till the next period starts. This is explained in the next illustration.
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The Temperature Indicator in Natural Family Planning
Normal body temperature levels vary in different women. But whatever the level, there will
be a sustained rise after ovulation until the next period.
The temperature must be taken:
 before getting out of bed in the
morning
 at the same time each day
and the readings recorded on a
special chart.
The temperature will stay at a
lower level until ovulation.
After ovulation, it will rise to a
higher level (about 0.3 centigrade)
10—16 days later, it will fall back
to the low level and the period
will start (top illustration).
If pregnancy is achieved (lower
illustration), the temperature
remains up (for nine months in
fact).
Special rules need to be learnt on
how to take and record
temperature, to make it work.
(See HOW TO CHART tutorial)
Statistical success of the Temperature Method: When a temperature chart is properly kept
and 3 genuinely raised temperatures have been recorded, the rest of the cycle is infertile,
with the same surprise pregnancy rate as female sterilisation—that is, virtually zero.
Limitations of the Temperature Method
 Body temperature can be disturbed by various factors
(illness, alcohol, oversleeping etc. See Guidelines).
 It gives no warning of when fertility begins.
 It only confirms when ovulation is over.
It was not until the 1960’s that the mucus symptom was understood and publicised by Drs.
Evelyn and John Billings. It came to be included in temperature charting to give a
combined double-check approach.
This combined approach is called the Sympto-Thermal Method, and this is the approach
preferred by the Fertility Education Trust.
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Other minor signs related to the fertility cycle
There are many other natural signs that women recognise in their cycle:



many women get pains around the time of ovulation, in their side or groin.
breast fullness or even tenderness is common after ovulation, sometimes
intermittent, sometimes lasting solidly till the start of the period.
pre menstrual tension, or PMS – pre-menstrual syndrome– is a nuisance side effect
of the cycle for many women. Those who suffer from it are better advised to seek
nutritional advise than resort to medication.
The effects of stress and other factors on a woman’s cycle
Some women have incredibly regular cycles, no matter what stresses occur in their lives.
For others, exams, a driving test, or an interview at the end of the month can lengthen a
cycle by two weeks! This illustration shows some of the more common situations that can
interfere with the hormone signals, usually causing the cycle to run late.
Stress and anxiety are the most
common causes of cycle disturbance.
Bereavements, illness in the family or
in the woman herself can also play a
part.
A variety of drugs, medicines, herbal
remedies can interfere with the normal
pattern of hormone activity, resulting
in cycle changes.
Air travel causes many cycles to run
late, as does poor diet, excessive
exercise and weight training. When
the training is extreme, the cycles can
stop completely as seen in competing
athletes.
Breastfeeding and the pre-menopause
phase involve specific hormone
changes which inevitably affect a
woman’s cycles.
Once a woman knows her symptoms,
she can recognise these unusual
events and their effects on her cycle
without any sense of alarm or
concern.
Please read all the other sections of the web site and then contact the Natural family
Planning Teachers’ Association via the email link in the “CONTACT” tab.
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